Nanoporous Anion Exchange Membranes for Solar Fuel Generators

Solar photoelectrochemical cells utilize sunlight, water (and sometimes also CO2), and convert them into hydrogen or other hydrocarbons by splitting water into hydrogen and oxygen (and optionally reacting the hydrogen with CO2 to form synthesis gas with CO2). This method is one of the most attractive technologies to produce carbon neutral or carbon negative fuels for transportation. However, to make photoelectrochemical cells effective, a durable separator is needed. Specifically, an anion exchange membrane (AEM) with excellent chemical stability in a strongly alkaline environment, good photo-stability, good hydroxide conductivity, and one that is mechanically robust to limit hydrogen and oxygen crossover. This project will develop an anion exchange membrane made from a very thin self-assembled, ionically conductive, nanoporous, and crosslinked polymer layer that can be coated onto a microporous expanded PTFE support membrane (ePTFE). The nanoporous polymer will consist of both chemically stable and ionically conductive domains that self-assembles into complex nanostructures with interconnected ionic channels, allowing hydroxide ions to travel across the membrane. The polymer material will also be highly crosslinked to prevent membrane swelling and to resist hydrogen and oxygen crossover. The ion-conducting region will contain stablilized imidazolium functional groups, which can resist hydroxide and UV attack and, thus, can operate at pH 14 under sunlight exposure for extended periods, as required for solar photoelectrochemical cells. In Phase I, TDA will prepare, characterize, and test these nanoporous AEMs in photoelectrolysis cells. The geometry of the polymerizable surfactants will be modified and characterized to study the structure-property relationships of the self-assembled materials. We will measure the mechanical properties, ion exchange capacity, hydroxide conductivity, photo-stability, and correlate them to the resulting nanoporous structure. The membrane processing methods will also be varied. Process-structure-property relationships will then be used to guide our development of these new membrane materials as highly selective anion-exchange membranes. Commercial applications include photoelectrochemical cells for long-duration hydrogen generation (to manufacture hydrogen for use in fuel cells, synthesis gas-to-transportation fuels, and back-up power systems).